1. Field of the Invention
The present invention relates to the production of alkylene carbonate, and especially to the removal of residual alkylene oxide from the alkylene carbonate by inert gas stripping.
2. Description of the Prior Art
It is known to form an alkylene carbonate by the reaction of an alkylene oxide with carbon dioxide. See, for example, U.S. Pat. Nos. 2,773,070, 2,873,282, 4,786,741, 5,179,214, 5,283,356 and the like. Appropriate catalysts and reaction conditions are known and taught, for example, in the above references.
A problem which has been associated with prior practices has been contamination of the product alkylene carbonate with unreacted alkylene oxide. One method heretofore employed for reducing the alkylene oxide level in product alkylene carbonate has been to continue the reaction between alkylene oxide and carbon dioxide until the alkylene oxide is essentially consumed. However these procedures involve excessive reaction times since the reaction slows considerably at low alkylene oxide concentrations.
It has been proposed, for example in U.S. Pat. Nos. 5,179,214 and 5,283,356 that in a continuous reaction system the alkylene oxide and carbon dioxide be introduced into a continuous reactor containing catalyst with continuous recirculation of a portion of the reaction mixture and continuous withdrawal and flashing of another portion of the reaction mixture to remove unreacted alkylene oxide and carbon dioxide; the removed materials can be compressed and returned to the reaction mixture. The residue from flashing is distilled to separate product carbonate from the catalyst solution. These procedures are not effective in reducing the level of unreacted alkylene oxide in the alkylene carbonate sufficiently without the necessity of elaborate and expensive distillation procedures. A problem in continuous systems has been that higher temperatures are employed than in batch systems in order to reduce unreacted alkylene oxide levels but this has the disadvantage of forming light and heavy impurities. Elaborate alkylene oxide distillation procedures are required to produce satisfactory product.